Phased array antennas are commonly used in satellite, electronic warfare, radar and communication networks. A phased array antenna includes a plurality of antenna elements and respective phase shifters that can be adjusted for producing a focused antenna beam steerable in a desired direction. A scanning phased array antenna steers or scans the direction of the RF signal being transmitted therefrom without physically moving the antenna. Likewise, the scanning phased array antenna can be steered or scanned without physically moving the antenna so that the main beam of the phased array antenna is in the desired direction for receiving an RF signal. This enables directed communications in which the RF signal is electronically focused in the desired direction.
Regardless of the chosen array geometry, it is required that the signal along each path between a signal source and the antenna elements have a controlled phase and magnitude to form a desired antenna pattern. This is achieved by controlling signal power division ratios and the phase shift in the electrical transmission path between the signal source and each antenna element. A structure which performs this function is generally referred to as an antenna feed.
In a conventional series feed network, a series of antenna elements are connected in a single transmission line with a built in phase progression between the antenna elements. The phase progression is determined in part by the length of the transmission line (physical path length) between successive antenna elements. The phase of the signal at each element is related to the electrical path length between antenna elements. The electrical path length, expressed in wavelengths, changes with frequency for a fixed physical path length. Therefore, the phase progression between antenna elements in a series feed varies with frequency.
Conventional methods of open loop calibration suffer from an inability to accurately model temperature gradient. Degradation of phase control causes defocusing of the antenna beam. Such conventional methods include making measurements on ground and loading information into a table. However, earth conditions are not the same as conditions at the antenna station. Also, sunlight obscuration and other conditions limit the accuracy of such an approach. Another method involves calibrating the phase using a known target. However, again, the sun angle changes and obscuration limit time that accuracy of the measurement is valid.